'\"
'\" Copyright (c) 1990 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
.TH Tk_CreateEventHandler 3 "" Tk "Tk Library Procedures"
.so man.macros
.BS
.SH NAME
Tk_CreateEventHandler, Tk_DeleteEventHandler \- associate procedure callback with an X event
.SH SYNOPSIS
.nf
\fB#include <tk.h>\fR
.sp
\fBTk_CreateEventHandler\fR(\fItkwin, mask, proc, clientData\fR)
.sp
\fBTk_DeleteEventHandler\fR(\fItkwin, mask, proc, clientData\fR)
.SH ARGUMENTS
.AS "unsigned long" clientData
.AP Tk_Window tkwin in
Token for window in which events may occur.
.AP "unsigned long" mask in
Bit-mask of events (such as \fBButtonPressMask\fR)
for which \fIproc\fR should be called.
.AP Tk_EventProc *proc in
Procedure to invoke whenever an event in \fImask\fR occurs
in the window given by \fItkwin\fR.
.AP ClientData clientData in
Arbitrary one-word value to pass to \fIproc\fR.
.BE
.SH DESCRIPTION
.PP
\fBTk_CreateEventHandler\fR arranges for \fIproc\fR to be
invoked in the future whenever one of the event types specified
by \fImask\fR occurs in the window specified by \fItkwin\fR.
The callback to \fIproc\fR will be made by \fBTk_HandleEvent\fR;
this mechanism only works in programs that dispatch events
through \fBTk_HandleEvent\fR (or through other Tk procedures that
call \fBTk_HandleEvent\fR, such as \fBTk_DoOneEvent\fR or
\fBTk_MainLoop\fR).
.PP
\fIProc\fR should have arguments and result that match the
type \fBTk_EventProc\fR:
.CS
typedef void \fBTk_EventProc\fR(
        ClientData \fIclientData\fR,
        XEvent *\fIeventPtr\fR);
.CE
The \fIclientData\fR parameter to \fIproc\fR is a copy of the \fIclientData\fR
argument given to \fBTk_CreateEventHandler\fR when the callback
was created.  Typically, \fIclientData\fR points to a data
structure containing application-specific information about
the window in which the event occurred.  \fIEventPtr\fR is
a pointer to the X event, which will be one of the ones
specified in the \fImask\fR argument to \fBTk_CreateEventHandler\fR.
.PP
\fBTk_DeleteEventHandler\fR may be called to delete a
previously-created event handler:  it deletes the first handler
it finds that is associated with \fItkwin\fR and matches the
\fImask\fR, \fIproc\fR, and \fIclientData\fR arguments.  If
no such handler exists, then \fBTk_HandleEvent\fR returns
without doing anything.  Although Tk supports it, it's probably
a bad idea to have more than one callback with the same \fImask\fR,
\fIproc\fR, and \fIclientData\fR arguments.
When a window is deleted all of its handlers will be deleted
automatically;  in this case there is no need to call
\fBTk_DeleteEventHandler\fR.
.PP
If multiple handlers are declared for the same type of X event
on the same window, then the handlers will be invoked in the
order they were created.
.SH KEYWORDS
bind, callback, event, handler
